The rotation part of a transmission or the like has a thrust roller bearing mounted thereon so as to support the thrust load applied to the rotation part or the like. FIG. 7 shows one example of a thrust roller bearing used for such a rotation part, that is described in Patent Document 1 (Japanese Patent Application Publication No. H 08-109925). This thrust roller bearing 1 is constructed with a plurality of rollers 2 (including needles) arranged in a radial direction, a cage 3 that retains the rollers 2, and a pair of races 4a and 4b that hold a plurality of the rollers 2 from opposite sides. The cage 3 comprises a first cage element 5 and a second cage element 6 each made in an overall annular shape with a C-shape cross section, assembled together with a hollow therebetween, with the same number of pockets 7 as the rollers 2 arranged in a radial pattern as shown in FIG. 8.
The first cage element 5 is made by applying plastic working such as press working to a metal sheet such as a steel sheet or the like, and comprises a first inside cylindrical portion 9 and a first outside cylindrical portion 10 formed concentric with each other on the inner and outer circumferences of a first ring portion 8 thereof. Moreover, first through holes 11 of rectangular shape, being long in the respective radial directions, are provided in a plurality of places around the circumferential direction of the first ring portion 8 for constructing the pockets 7. Furthermore, the second cage element 6 is also made by applying a plastic working process such as press working to a metal sheet such as a steel sheet, and comprises a second inside cylindrical portion 13 and a second outside cylindrical portion 14 formed concentric with each other on the inner and outer circumferences of a second ring portion 12 thereof. Moreover, second through holes 15 of rectangular shape, being long in the respective radial directions, are provided in a plurality of places around the circumferential direction of the second ring portion 12 for constructing the pockets 7. The first cage element 5 and the second cage element 6 each having such constructions, are combined so that the second outside cylindrical portion 14 is fitted into the radial inside of the first outside cylindrical portion 10, and the second inside cylindrical portion 13 is fitted onto the radial outside of the first inside cylindrical portion 9, in a condition with the first through holes 11 and the second through holes 15 matched with each other in relation to the axial direction. Moreover, by folding the tip edge of the first inside cylindrical portion 9 radially outwards, the parts are prevented from separating from each other.
Furthermore, the races 4a and 4b are each made in a ring shape from a metal plate of sufficient hardness. Short cylindrical folded up walls 16a and 16b are respectively formed on the inner circumference edge of the race 4a, which is commonly known as the inner race (left hand in FIG. 7), and the outer circumference edge of the other race 4b, which is commonly known as the outer race (right hand in FIG. 7). Moreover, by folding radially outward a plurality of places on the tip edge of the folded up wall 16a, and by folding radially inward a plurality of places on the tip edge of the folded up wall 16b, locking portions 17a and 17b are formed. The locking portions 17a and 17b, and the inner circumference edge or the outer circumference edge of the cage 3 are engaged with each other so that the component parts of the thrust roller bearing 1 are inseparably joined to each other.
As shown for example in FIG. 7, the thrust roller bearing 1 constructed as described above is mounted onto a rotating part where a thrust load occurs, in a state where the folded up wall 16b formed on the outer circumference of the aforementioned race 4b called an outer race, is fitted into a concave cylinder shaped retaining portion 19 formed in a casing 18. In this state, the right hand surface of the race 4b makes contact with a back surface 19a of the retaining portion 19, and the left hand surface of the other race 4a makes contact with an end surface 20a of an opposite member 20. As a result, the casing 18 rotatably supports the opposite member 20, and the thrust load acting between both members 20 and 18 is thus supported. There is also a case where, the back surface 19a or the end surface 20a serve as a raceway surface, and one or both of the races 4a and 4b are omitted.
During use of the thrust roller bearing 1 described above, a force based on centrifugal force and directed in the radial outward direction of the cage 3, is applied to the rollers 2. Due to this force, of the two axial end surfaces of the rollers 2, the outside end surface 21 on the radial outside of the cage 3 is pressed against outside circumference edge portions 22a and 22b on the radial outside of the cage 3, among the circumference portions of the first and second through holes 11 and 15 that constitute the pockets 7. However, this is not to say that the outside end surface 21 is uniformly pressed against the outside circumference edge portions 22a and 22b. In actuality, the outside end surface 21 is pressed against and is in sliding contact with either one of the outside circumference edge portions 22a, or 22b, depending on manufacturing error or displacement of the cage 3 in the axial direction (right and left direction in FIG. 7).
A face pressure P of the sliding part becomes greater as the operating rotation speed of the thrust roller bearing 1 becomes higher and the centrifugal force becomes greater. Furthermore, since the sliding part is in a position distanced from the central axis of the rollers 2, a sliding velocity V between the outside circumference edge portion 22a (or 22b) and the outside end surface 21 becomes greater to a degree. Accordingly, at the sliding part a PV value being the product of the face pressure P and the sliding velocity V, which is widely known as a parameter indicating an influence on wear, becomes greater. As a result, a concavity 23 due to wear may be formed on the outside circumference edge portion 22a (or 22b) as shown in FIG. 9. If the concavity 23 becomes large, the rollers 2 retained in the pockets 7 can slide into the back side of the first ring portion 8 (or the second ring portion 12) where the concavity 23 is formed, and rotation of the rollers 2 cannot be performed smoothly. At the same time, one side surface of the cage 3 (right hand surface of the first cage element 5, or left hand surface of the second cage element 6 shown in FIG. 7) is pressed against side surface of the race 4a or the race 4b which is the opposite surface, so that the resistance in relation to the relative rotation of the cage 3 with respect to the race 4a or the race 4b becomes greater. As a result, not only is the efficiency of machine equipment such as a transmission having the thrust roller bearing 1 assembled therein reduced, but in extreme cases the machine equipment may also become unable to operate normally due to damage such as seizing.
Wear that causes such disadvantages has become more likely to occur compared to heretofore, with the speeding up of the rotation speed of the rotating parts of a transmission, due to recent improvements in the performance of vehicles. Moreover, an investigation carried out by the present inventor has discovered that the wear is more likely to occur when the movement amount of the rollers 2 inside the pockets 7 becomes greater. This point is now described with reference to FIG. 10 and FIG. 11. As shown in FIG. 10, the roller 2 retained inside the pocket 7 of the cage 3 constructed from the combination of the first cage element 5 and second cage element 6, is displaced in the axial direction of the cage 3 (left/right direction in FIG. 10 and FIG. 11) inside the pocket 7 by an amount L as shown in FIG. 11. Moreover, this displacement amount L becomes greater when the distance between the rotation contact surface of the roller 2 and the inner peripheral edge of the pocket 7 becomes greater. The displacement amount L is measured by turning the cage 3 through 180 degrees in a condition with the central axis of the cage 3 vertically positioned. In a case taking the right hand surface in FIG. 11 as a reference surface for example, the roller 2 projects from the reference surface by an amount of ΔL1 (projecting amount=ΔL1), when this reference surface is at the bottom, and the roller 2 is retracted from the reference surface by an amount of ΔL2 (retracting amount=ΔL2) when the reference surface is at the top. The displacement amount L equals the sum of the projecting amount ΔL1 and the retracting amount ΔL2 (L=ΔL1+ΔL2).
Moreover, the result of the experiment carried out by the present inventor revealed that the displacement amount L greatly affects the occurrence of wear. In the thrust roller bearing used for this experiment, the diameter of each roller 2 was 2 mm, the axial length was 4.2 mm, and the pitch circle diameter was 59.465 mm. A thrust roller bearing such as this was rotated for 6 hours at 10200 min−1 with a 1455 N thrust load applied, and the temperature of the lubrication oil at 125° C. The results of the experiment carried out under these conditions are shown in FIG. 12. In FIG. 12, the upper line with the reference symbol “♦” denotes the projecting amount ΔL1, and the bottom line with the reference symbol “▪” denotes the retracting amount ΔL2. Moreover, of twenty one pockets numbered 1 to 21 to indicate their positions, an amount of wear to cause the roller 2 to slide, occurred in the pocket in position 4 and the pocket in position 20, while such an amount of wear did not occur in the other pockets. Similar experimental results were also obtained for a thrust roller bearing with the diameter of each roller 2 of 3 mm, the length 5.8 mm, and the pitch circle diameter 49.87 mm.
The displacement amount L of the roller 2 (=ΔL1+ΔL2) was greater in the pockets at positions 4 and 20 than in the other pockets, and this is believed to have caused the wear. That is, a greater displacement amount L leads to a greater likelihood of displacement of the roller 2 inside the pocket 7, and a greater likelihood of so-called skew where the rotation center of the roller 2 and the radial direction of the cage 3 become out of alignment. In the case where such skew does not occur, and the rotation center of the roller 2 matches with the radial direction of the cage 3, the direction of the movement of the roller 2 accompanying the rotation thereof, matches the rotation direction of the cage 3, and there is no component in the radial direction of the cage 3. On the other hand, if skewing occurs, a component in the radial direction of the cage 3 occurs corresponding to the direction of the movement of the roller 2 accompanying the rotation thereof. Moreover, if this radial direction component is directed in the radial outward direction of the cage 3, this adds to the force based on the centrifugal force, so that the face pressure on the sliding part between the outside end surface 21, and the outside circumference edge portions 22a (or 22b) becomes higher, and this is understood to cause the aforementioned wear.
A structure is described in the Patent Document 2 (Japanese Patent Application Publication No. 2003-172346) in which at an outside end portion of the cage, metal plates that constitute a pair of cage elements, are overlapped in the axial central portion of the cage. According to such a structure described in Patent Document 2, sliding of a roller due to the above wear is believed to be prevented. However, in the case of the structure described in Patent Document 2, since the surface area of the outer peripheral surface of the cage is narrow, there is a possibility of the opposite surfaces facing this outer peripheral surface becoming worn due to friction with the outer peripheral surface, thus limiting the places in which it may be used. Moreover, since the portion where the metal plates are overlapped needs to be spot-welded, an increase in manufacturing cost may be expected.
Furthermore, a thrust roller bearing having a cage comprising a single metal plate formed with a corrugated cross sectional shape is described in Patent Document 3 (Japanese Patent Application Publication No. 2002-206525). According to such a structure described in Patent Document 3, the abovementioned sliding of the roller due the wear is also believed to be prevented by appropriately designing the positions for forming the pockets. However, the structure described in Patent Document 3 greatly differs from the intended structure of the invention of the present application. Therefore, a structure that achieves the prevention of wear cannot be obtained using equipment conventionally provided for manufacturing the cage 3 having the structures shown in FIG. 7 and FIG. 10. The same applies to the structure described in Patent Document 2.
[Patent Document 1] Japanese Patent Application Publication No. H08-109925
[Patent Document 2] Japanese Patent Application Publication 2003-172346
[Patent Document 3] Japanese Patent Application Publication 2002-206525